Fair Value Measurements (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 1,058,714,000		$ 1,087,377,000
Other assets	26,442,000		32,944,000
Other liabilities	25,219,000		28,213,000
Debt	1,134,695,000		1,384,105,000
Long-term debt, fair value	1,201,347,000		1,468,618,000
Net investment income | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains recognized in earnings	(60,100,000)	$ 1,300,000
Other income (loss) | Other assets and (liabilities)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains recognized in earnings	0	$ 0
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	1,058,714,000		1,087,377,000
Other assets	3,078,000		21,668,000
Fair Value, Measurements, Recurring | Private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	240,277,000		271,047,000
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	1,223,000		4,731,000
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	72,620,000		74,634,000
Other assets	1,223,000		4,731,000
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	72,620,000		74,634,000
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	1,223,000		$ 4,731,000
Significant Unobservable Inputs (Level 3) | External valuation model | Fair Value, Measurements, Recurring | Private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	1,822,000
Significant Unobservable Inputs (Level 3) | Internal valuation model | Fair Value, Measurements, Recurring | Private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	70,798,000
Significant Unobservable Inputs (Level 3) | Internal valuation model | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	1,200,000
Significant Unobservable Inputs (Level 3) | Internal valuation model | Fair Value, Measurements, Recurring | Assumed reinsurance contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	8,556,000
Significant Unobservable Inputs (Level 3) | Internal valuation model | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	11,011,000
Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Miscellaneous other investments	$ 199,563,000
U.S. treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	0.40%		1.70%
Agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.00%		2.10%
Non-U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	0.80%		1.60%
Non-U.S. government-backed corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.20%		2.00%
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	3.80%		3.00%
Agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.40%		2.50%
Weighted average life	3 years 6 months		4 years 10 months 24 days
Non-agency prime residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	5.60%		3.30%
Weighted average life	4 years 4 months 24 days		4 years 9 months 18 days
Alt-A non-agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	6.30%		3.80%
Weighted average life	5 years 10 months 24 days		6 years 3 months 18 days
Commercial mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	3.10%		2.60%
Weighted average life	5 years 9 months 18 days		5 years 8 months 12 days
Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	4.40%		3.30%
Weighted average life	3 years		3 years 2 months 12 days
Short term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	0.50%		1.60%
Private equity investments | Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets	7 years
Private equity investments | Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets	10 years
Private equity investments | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Miscellaneous other investments	$ 167,657,000
Senior secured bank loan funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Miscellaneous other investments	$ 22,600,000
Senior secured bank loan funds | Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets	4 years
Senior secured bank loan funds | Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets	5 years
Senior secured bank loan funds | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Miscellaneous other investments	$ 22,579,000
Hedge funds | Fair Value Measured at Net Asset Value Per Share
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Miscellaneous other investments	$ 9,327,000